1. Field of the Invention
This invention relates to an image forming apparatus having fixing means for fixing a toner image formed on a recording material such as a transferring sheet, photosensitive paper or electrostatic recording paper by a suitable image forming process such as an electrophotographic recording process or an electrostatic recording process.
2. Description of Related Art
In image forming apparatuses such as conventional copying machines and printers, in order to fix an unfixed toner image on a recording material, use is widely be made of a fixing apparatus for passing a recording material through a nip part formed by a fixing roller and a pressure roller for nipping and conveying the recording material, to thereby fix the toner image by heat and pressure.
FIG. 11 of the accompanying drawings shows a cross section of a conventional roller type fixing apparatus. In FIG. 11, the reference numeral 101 designates a fixing roller, and the reference numeral 102 denotes a pressure roller. These rollers are rotatively driven at a predetermined peripheral speed in the directions of arrows. These two rollers 101 and 102 are brought into pressure contact with each other with a predetermined pressure force, whereby there is formed a fixing nip part N for nipping and conveying a recording material 109 on which there is unfixed toner t.
The fixing roller 101 has a mandrel 103 made of a metal or the like and formed into a cylindrical shape, and a heater 105 such as a halogen heater is inserted and disposed in the mandrel 103. This heater 105 generates heat by electrical energization and warms the mandrel 103. Further, heat is transmitted from the heater 105 to the mandrel 103 to thereby warm the surface of the fixing roller 101. A main temperature sensor 107 disposed correspondingly to the substantially length wisely central position of the fixing roller 101 for detecting the surface temperature of the fixing roller 101 detects the surface temperature of the fixing roller 101, and the detected temperature is inputted to a control circuit 110. The control circuit 110 control an electric power supplying circuit 111 for the heater 105 so that the detected temperature from the main temperature sensor 107 may become a preset temperature. Also, the reference numeral 108 designates a sub-surface temperature sensor disposed on an end portion (non-sheet passing area) of the fixing roller 101, and it serves to obviate the fixing apparatus from going wrong by the fixing roller being excessively heated due to the trouble or the like of the main temperature sensor 107.
FIG. 12 of the accompanying drawings shows a timing chart of a copying machine or a printer during the closing of a power switch in an example of the prior art. The reference numeral 201 represents the print starting state of the printer. From power-on timing 203, the printer enters a warming-up operation for increasing the surface temperature of the fixing roller to a fixing temperature, and at timing 204, it comes to a printing capable state.
On the other hand, the reference numeral 202 denotes a timing chart showing the rotation state of the fixing roller 101 and pressure roller 102 of the fixing apparatus shown in FIG. 11. The fixing roller 101 and the pressure roller 102 continue heating while being rotated until they come to the printing capable timing 204 at which the detection value of the main temperature sensor 107 reaches a print starting temperature, in order to uniformly transmit the heat to the entire fixing roller and to transmit the heat of the fixing roller to the pressure roller.
In such a construction, depending on the state of the fixing roller or the pressure roller during the closing of the power switch, when the temperature of the fixing roller comes to a printing temperature and the printing capable timing 204 is reached, there is a case where the surface of the pressure roller is warmed but the entire pressure roller is not warmed. As a result, even after the printing capable timing 204 has been reached, there is a case where the heat from the surface of the pressure roller is radiated in some time and a fixing property is not satisfied. Therefore, as described in Japanese Patent Application Laid-Open No. 10-142999, after the printing capable timing 204, the pressure roller is further rotated while being controlled at a predetermined temperature for a predetermined time (T), whereby the entire pressure roller can be warmed. In Japanese Patent Application Laid-Open No. 5-333624, there is described a construction in which a pressure roller has a heater such as a halogen heater-therein, and even in such a construction, if usable electric power is limited, sufficient electric power cannot be supplied to the heater in the pressure roller and therefore, the pressure roller cannot be sufficiently warmed by this heater alone and therefore, in some cases, the construction of Japanese Patent Application Laid-Open No. 10-142999 is adopted. However, in a method of making the rotation time after the printing capable timing 204 constant, there is a case where the pressure roller cannot be sufficiently warmed depending on environment, or there is the problem that in spite of the pressure roller being sufficiently warm, idle rotation is uselessly effected and consumed electric power becomes great. On the other hand, in Japanese Patent Application Laid-Open No. 62-87909, there is described a method of determining the idle rotation time during the raising operation from a temperature rise rate during some of a rising time. However, judging from some of the time, the degree to which the pressure roller is warmed cannot be judged accurately when the surface of the pressure roller is warm but the mandrel portion is cold. Therefore, it is desirable to grasp how much the pressure roller has been warmed during the raising time, and determine the idle rotation time.